Order Edit 'Undefined Variable: json' Error in 2.0.3.1

Error in OC version 2.0.3.1 when editing order status: I had the error several months ago and resolved it by creating and assigning a new API user, but this no longer works.

Is there a work around for this? I desperately need to update orders and have already checked API database settings, created new API, editing /public_html/admin/controller/sale/order.php, and other steps found on the forum.

I have full SSL running on the site, not sure if this makes a difference, but have been editing orders successfully for many months without any problems until recently, and I haven't changed any settings or files so don't know why the error has suddenly returned.

As you have already done api user point, you can check this points below :

1) Check if site is not in maintenance mode.
2) if admin is non https and front end is https, it can also be cause of problem.
3) No password protection added Ex: .htaccess

We also have an extension for changing order status directly from list and some more features :
Improved Order Page

If you need help you can also contact us directly here:
http://support.cartbinder.com/open.php

1) Site is not in maintenance mode.
2) Admin is https and front end is https (site is fully https).
3) How do I check this?: No password protection added Ex: .htaccess

Okay if you dont know about this it means you don't have one. All 4 points are okay for you. Seems errors is something different. If you need help, you can create a help ticket here:
http://support.cartbinder.com/open.php